The regulation of the cellular procoagulant tissue factor (thromboplastin) and the cellular fibrinolytic system (both plasminogen activator and fibrinolytic inhibitor(s)) of cultured cells is being studied. Dexamethasone is known to suppress fibrinolytic activity. This may occur by either a suppression of plasminogen activator production or an enhanced synthesis of a fibrinolytic inhibitor(s). These alternative mechanisms and the nature of the fibrinolytic inhibitor are being examined. Platelets appear to be the physiological source of a substance which can simultaneously stimulate tissue factor and fibrinolytic inhibitory activity in several cell types. This platelet-derived material is being purified and its chemical and biological properties will be studied. It is not clear how exogenous drugs or naturally occuring modulator substances, such as the platelet-derived factor, act to regulate procoagulant or fibrinolytic activities. Prostaglandins play a regulatory role in a number of cellular processes including several involved in hemostasis. Studies are under way to explore their possible involvement in the control of tissue factor and fibrinolytic activity.